CODEWITHSUNDEEP
javascriptarrays
jairajs89
jairajs89

Reputation: 4825

What's the fastest way to merge Array's in JavaScript where comparisons are expensive?

I want to merge two Array's in JavaScript. Unfortunately, comparisons of the particular data I am using is expensive. What is the best algorithm to merge my to lists with the least amount of comparisons?

EDIT: I should note that the two Array's are sorted and I would like the merged content to be sorted and have only unique values.

EDIT: By request I will give you my current code, but it really doesn't help..

// Merges arr1 and arr2 (placing the result in arr1)
merge = function(arr1,arr2) {
    if(!arr1.length) {
        Array.prototype.push.apply(arr1,arr2);
        return;
    }
    var j, lj;
    for(var s, i=0, j=0, li=arr1.length, lj=arr2.length; i<li && j<lj;) {
        s = compare(arr1[i], arr2[j]);
        if(s<0) ++i;
        else if(s==0) ++i, ++j;
        else arr1.splice(i,0, arr2[j++]);
    }
    if(j<lj) Array.prototype.push.apply(arr1, arr2.slice(j));
};

Upvotes: 3

Views: 2320

Answers (3)

glenn_echo
glenn_echo

Reputation: 1

This seems to do it for me using maps to do comparisons instead of a comparison function. It results in more loops though which is often more expensive than the comparison. This was used to right join arrays of objects that had a name attribute. It also had the unique problem of having to modify the existing array vs. creating a new array and assigning. Some reference thing with D3 and force nodes.

function merge(right,left) {
    var map_right = {},
        map_left = {};
    right.forEach(function(n) {
        map_right[n.name] = n;
    });
    left.forEach(function(n) {
        map_left[n.name] = n;
    });
    for(var n in map_left) {
        if (map_right[n] === undefined) {
            left.splice(nodes.indexOf(map_b[n]),1);
        }
    }
    right.forEach(function (n) {
        if (map_left[n.name] === undefined) {
            left.push(n);
        }
    });
}

Upvotes: 0

TmEllis
TmEllis

Reputation:

Here is a function to make the array unique:

Array.prototype.unique = function () {
    var r = new Array();
    o:for(var i = 0, n = this.length; i < n; i++)
    {
        for(var x = 0, y = r.length; x < y; x++)
        {
            if(r[x]==this[i])
            {
                continue o;
            }
        }
        r[r.length] = this[i];
    }
    return r;
}

To merge Arrays you would do:

var MainArray= arr1.concat(arr2);

Then to remove duplicates:

var MainArray= MainArray.unique();

Then to sort:

var MainArray= MainArray.sort(); //Or you own sort if you have one

I think this sounds along the lines you were looking for, but it might depend on the amount and type of data.

Upvotes: 1

Anurag
Anurag

Reputation: 141879

This is exactly what merge sort does in the merging step with a minor addition that duplicate elements will be dropped off. Here's the algorithm:

function merge(left,right)
    var list result
    while length(left) > 0 and length(right) > 0
        if first(left) ≤ first(right)
            append first(left) to result
            left = rest(left)
        else
            append first(right) to result
            right = rest(right)
    end while
    if length(left) > 0 
        append left to result
    else  
        append right to result
    return result

Upvotes: 1

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